1. Field of the Invention
The present invention relates to a wavelength division multiplexing transmission system, a wavelength division multiplexing transmission apparatus, and an optical signal noise ratio calculation method used for the system and apparatus.
2. Description of the Related Art
Due to an increased volume of communicated information, a high capacity and low cost optical fiber communication system has been developed. In such a system, a wavelength division multiplexing (WDM) method for multiplexing multiple wavelengths and transmitting the multiplexed signal has been adopted and the degree of wavelength-multiplexing tends to increase higher and higher in recent years.
Channel spacing, an index of the degree of wavelength-multiplexing, has been standardized by the International Telecommunications Union Telecommunications Standardization Sector (ITU-T). It is known that a standard WDM system wavelength-multiplexes a signal whose the signal transmission capacity per one channel is a 10-gigabit per second (Gbps) with a 100-gigahertz (GHz) (approximately 0.8 nano meters (nm)) spacing or a 50-GHz (approximately 0.4 nm) spacing.
In WDM systems, an erbium doped fiber amplifier (EDFA) is commonly used as a repeater to offset optical fiber line loss. In a system employing EDFA, amplified spontaneous emission (ASE) is generated becoming noise causing bit error rate (BER) increases. As such, the optical signal noise ratio (OSNR) evaluation becomes important.
Since the WDM system transmits multiple channels simultaneously, the OSNR for the receiving end (after transmission) of each channel differs for each channel. Additionally, the BER of each channel also varies. As such, the quality of transmission among channels becomes unequal. Hence, to optimize the transmission level of each channel such that the transmission quality becomes equivalent, a pre-emphasis method is commonly employed.
In the pre-emphasis process, since the level of each channel on the transmission side is determined based on the OSNR, the OSNR must be accurately measured. A method for measuring the spectrum of the signal and the ASE component by using a spectrum analyzer and calculating the OSNR based on the measurement results provides a relatively high accuracy.
The following patent document proposes an OSNR measurement apparatus and an OSNR measurement method that can accurately measure the OSNR in an optical fiber communication of high bit rate and high-density channel spacing.
[Patent document 1] Japanese Patent Application Laid-Open Publication No. 2008-85883.
In recent years, due to an increasing demand for a much larger amount of the transmission capacity, the transmission wavelength becomes narrower and the bit rate of the transmission wavelength becomes still higher. Hence, it is hard to acquire a wavelength point where only the noise component can be measured without suffering from the effect of the modulated sideband component of the transmission signal so that an accurate noise optical power cannot be obtained. Thus there exists such a problem that the OSNR cannot be accurately measured under these circumstances.